Surfactants are used in a variety of applications. For example, surfactants are used commercially for cleaning manufactured items, removing paints, chemical processing, for use in emulsion polymerization, solubilizing drugs, purifying proteins, and various bioanalytical applications.
One particular bioanalytical application that uses surfactants is sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). In the past three decades, SDS-PAGE has been widely used as a simple and relatively rapid tool for analysis and purification of large molecules such as proteins (U. K. Laemmli, Nature 227, 680–685, 1970). Sodium dodecylsulfate (SDS) is an anionic surfactant that denatures proteins by forming a stable complex. Upon denaturation, SDS binds to most proteins and peptides in a constant weight ratio of about 1.4:1. As a result, the SDS-protein complexes have almost identical charge densities and therefore migrate in a polyacrylamide gel according to molecular weight. If the gel is of the correct porosity, a plot of log Mw vs. relative mobility, Rf, results in a linear relationship. The band intensity after staining is a rough indicator of the amount present in the sample. When coupled with another electrophoretic technique, isoelectricfocusing, SDS-PAGE can separate complex mixtures into several hundred discrete components.
The ability to estimate the size and amount of a protein has led to various applications of SDS-PAGE. However, there are some drawbacks to the technology. For example, it is very difficult to use mass spectrometry to monitor and analyze samples from SDS-PAGE separations because SDS interferes with the sensitivity of mass spectrometry detection. Furthermore, it is very difficult to separate SDS from SDS/protein complex since SDS is a surfactant that forms emulsions.
Several approaches have been tried to solve these problems. Non-ionic surfactants, such as octyl β-glucopyranoside, have been used for mass spectrometric applications (P. Dainese Hatt, M. Quadroni, W. Staudenmann, and P. James, Eur. J. Biochem. 246, 336–343, 1997). However, the electrophoretic separation still requires SDS, and a time-consuming surfactant exchange step is needed.
Another approach is electroelution or electroblotting from the polyacrylamide gel onto a PVDF or nitrocellulose membrane. However, this approach often leads to significant loss in protein recovery.
Other approaches that have been tried are also time-consuming and may lead to significant protein loss are: protein precipitation with guanidium chloride (J. E. Schively, in Methods of protein microcharacterization; J. E. Schively, Ed., Humana Press, Clifton, N.J., 1986, p. 41.), ion-pair reagents (W. H. Koningsberg and L. H. Henderson. Methods Enzymol. 91, 254, 1983), liquid—liquid extraction (P. Davidsson, A. Westman, M. Puchades. C. L. Nilsson, and K. Blennow, Anal. Chem. 71, 642–647, 1999) and reversed-phase HPLC (H. Kawasaki and K. Suzuki, Anal. Biochem. 186, 264, 1990).